1. Field of the Invention
The present invention relates to an image display method of displaying images obtained by radiography and so on, and relates to an image display apparatus using the same.
2. Description of a Related Art
Photography using radiation (X-rays, α-rays, β-rays, γ-rays, electron rays, ultraviolet rays and so on) has hitherto been used in a variety of fields, and especially in the medical field it becomes one of indispensable measures for diagnoses. X-ray radiography has had a lot of improvements since its first realization and now combining fluorescent screen and X-ray film is the mainstream. On the other hand, with various digitalized apparatuses such as X-ray CT, ultrasonic diagnosis, MRI and so on realized in recent years, diagnostic information processing systems are being structured in hospitals. With a lot of researches conducted to digitize X-ray images, radiography using photostimulable phosphor has been established and put into practical use to replace X-ray radiography.
Photostimulable phosphor (accumulative phosphor) is such a substance as follows. That is, the substance accumulates a part of the radiation energy when radiation is irradiated. After that, when excitation light such as visible light and so on is irradiated, the substance radiates photostimulated luminescence light in accordance with the accumulated energy. Existence of photostimulable phosphor has been known so far. The radiography using photostimulable phosphor is as follows. That is, a radiation image of an object like a human body is photographed and recorded on a sheet coated with photostimulable phosphor. Since photostimulated luminescence light is produced when the photostimulable phosphor sheet is scanned by excitation light such as a laser beam and so on, image data can be obtained by reading the light photo-electrically. After proper processing of the image data, a radiation image can be shown as a visible image on a display such as a CRT or on a film printed by a laser printer and so on.
The radiography compares with the conventional X-ray radiography in photographic sensibility and image quality. For example, compared with the conventional X-ray radiography, an exposure area is exceedingly wide and a response of photostimulated luminescence light is almost in proportion to an exposure amount in all the exposure area. Therefore, even if an object is photographed by any amount of radiation, image signals having proper gradation can be obtained by finding the luminous area of the image and normalizing it. Also, with a proper process of image signals obtained by this way, the image having good quality can be produced under a variety of photographing condition. Moreover, it is possible to store a lot of image data set for a long time without deterioration of image quality because the image by this radiophotography can be stored as digitized data. Such a system storing images as image data sets will be useful to the development of medical diagnostic information system.
In the conventional medical radiography system using the photostimulable phosphor sheet, a radiation image is photographed on photostimulable phosphor sheet, after that, image data representing the radiation image recorded on the sheet is read through a reading apparatus. Then, a whole of the read image data is read into a processing unit included in an image processing apparatus or an image display apparatus, and then, the image data is performed processing operation. So that, such a system has a defect of taking long time to display the visible image since the system reads the image data.
To judge the quality of photography, it is important to display a visible image soon and check it. Then, it is possible to save trouble of photographing again when, for example, mistake of photographing is made. Therefore, it is required to display high quality images as soon as possible.
Japanese Patent No. 3038407 discloses a radiation image information reading apparatus which writes linear gradation processing condition into a look-up table for display use and displays an image on the basis of image data by using the look-up table for display use in response to reading of a radiation image. After that, the apparatus obtains gradation processing condition corresponding to the radiation image on the basis of frequency distribution of the image data, writes the obtained gradation processing condition into the look-up table for display use, and alters an image to be displayed.
The look-up table is a conversion table used to alter received data value, and here, is used to rectify the gradation of the radiation image. The look-up table corresponding to photographing condition is made to reproduce gradation like that of film images on the basis of the received image data or reproduce photographed images in optimum gradation which differs with portions to be photographed.
The above-mentioned apparatus makes it possible to successively display an image while receiving the image data. However, in the apparatus, the image is not displayed in good gradation until all image data is received because the look-up table is fixed while the image is successively displayed, and altered by calculating optimum gradation when all image data is received, then the image is redisplayed. Therefore, the image is sometimes displayed in low density, that is, whitish or conversely, in high density, that is, blackish, which is improper to judge the quality of photography.